Preparation of low sulfur content heavy fuel oils



United States Patent 3,324,028 PREPARATION OF LOW SULFUR CONTENT HEAVY FUEL OILS Harold Beuther, Gibsonia Township, Allegheny County,

and Bruce K. Schmidt, McCandless Township, Allegheny County, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware No Drawing. Filed Apr. 22, 1964, Ser. No. 361,893 3 Claims. (Cl. 208-89) This invention relates to the preparation of improved residual type fuels and especially to the preparation of residual type fuels meeting the specifications for No. 6 fuel oil, 3500 Redwood No. 1 fuel oil or the like, particularly the viscosity specifications and such sulfur specifications as may be imposed by purchasers and/or local statutes.

Residual petroleum fractions and certain heavy crudes, both of which have an API gravity below about 20, are products of relatively low value. They are too viscous to employ as low-grade fuel oils. Also their sulfur content is normally so high that these materials cannot be used as low-grade fuel in municipalities that have adopted maximum sulfur specifications for fuels burned in their jurisdiction. It has been proposed previously to subject such residual fractions to visbreaking in order to im prove their viscosity characteristics. This treatment forms a small amount of volatile materials of higher value and also intermediate boiling components, such as furnace oil, which ordinarily are left in the visbroken residual stock in order to decrease its viscosity. Nevertheless, the improvement in viscosity obtained by visbreaking is frequently inadequtae to satisfy the viscosity requirements for residual fuels. Furthermore, the removal of sulfur from visbroken oils is still a problem since the residual fuel produced by visbreaking characteristically has a higher sulfur content than the feed stock charged to the visbreaking process.

It also has been proposed previously to subject high sulfur content residualstocks to hydrodesulfurization, either to reduce sulfur alone or to reduce both sulfur and viscosity. See, for instance, Beuther et al. Preprint, American Petroleum Institute, Division of Refining, Houston, May 11, 1961, pages 1 to 9. This procedure as practiced commercially commonly involves operations oftwo general types, one being carried out at moderate hydrogen partial pressures of about 1000 p.s.i.g., the other at more elevated hydrogen partial pressures such as 2000 psig. and above. While the moderate pressure type of operation is effective to reduce sulfur content significantly, it does not ordinarily produce a marked change in viscosity or pour point. The higher pressure type operation is even more effective to reduce sulfur and, in addition, effects an appreciable reduction in viscosity. Nevertheless, high pressure hydrodesulfurization is not fully satisfactory for purposes of viscosity reduction, since such processes may require reduction of the sulfur content of a feed stock to an unnecessarily low and correspondingly costly level, in order to achieve the desired degree of viscosity reduction.

It has now been found that visbreaking of hydrodesulfurized residual oils, as opposed to either visbreaking or hydrodesulfurizing, results in the production of high quality, residual fuel base stocks of low viscosity, that have surprisingly low sulfur contents and low pour points, and that require unsually small quantities of cutter oils as diluents, without excessive hydrogen consumption or unnecessary desulfurization. The present invention, based on this discovery, relates to a combination process including as a first step, subjecting a residual hydrocarbon feed that has a gravity below about 20 API and a sul- 3,324,028 Patented June 6, 1967 fur content of at least about 2 percent to a catalytic hydrodesulfurization treatment at a hydrogen partial pressure in the range of about 400 and 2000 p.s.i.g. The temperature during hydrodesulfurization is in the range of about 700 and 850 F., preferably 750 to 850 F., and the space velocity is in the range of about 0.2 and 10, these conditions being so selected that the product from the hydrodesulfurization after removal of furnace oil and lighter hydrocarbons has a pentane-insolubles content which is the same as or below the pentane-insolubles content of the feed stock. Hydrodesulfurized residual hydrocarbon product is then subjected to a visbreaking operation in which the reaction conditions are so selected that no more than about 20 percent gasoline is formed.

Surprisingly, it has been found that visbreaking is remarkably effective in reducing the pour point and viscosity of hydrodesulfurized residual oils and in reducing sulfur, notwithstanding its relative lack of effectiveness in these respects when used with other feed stocks. The chemical mechanism by which the combination process of this invention functions has not been definitely established. However, available evidence suggests that complex molecules involving two or more cyclic nuclei partially linked together by sulfur, such as substituted dibenzothiophene-type molecules, may have their sulfur linkages removed during the hydrodesulfurization stage. The desulfurized molecules, which prior to desulfurization are resistant to cracking under visbreaking conditions, are considered to be more susceptible to cracking during visbreaking. The desulfurized and cracked molecules are less viscous than their original or desulfurized uncracked precursors. Accordingly, hydrodesulfurization of residual oils followed by visbreaking, if accompanied by these reactions, will result in a product having an unusually low pour point and requiring unusually small proportions of cutter (diluent) oil to reduce the viscosity to the levels required by typical residual fuel oil specifications.

The feed stock to the hydrodesulfurization stage of our process may be any petroleum stock containing residual materials, that has an API gravity below about 20 and that has a sulfur content of at least about 2 percent. Thus, the feed stock may be a whole crude which has an API gravity below about 20. While such low gravity crudes are unusual, they do exist and our process is applicable thereto. However, our process more commonly will be applied to a bottoms fraction of petroleum, i.e., one Which is obtained by distillation of a crude to remove lower boiling materials such as naptha, furnace oil, and/or gas oil. Such a bottoms fraction may be obtained by atmospheric and/ or vacuum distillation of the crude. Our invention is particularly applicable to the treatment of straight run residual fractions of crude petroleum having a boiling range above about 1000 F. The feed stock should be one which contains sulfur in such large amounts that partial removal is necessary. The sulfur content of petroleum stocks suitable for use in the present invention usually will be between about 2 and 8 percent. A high sulfur content in the feed stock is important as this indicates a significant number of molecules whose viscosity can be reduced by the combination process of this invention.

The hydrodesulfurization stage of our invention may be carried out utilizing any suitable hydrodesulfurization catalyst. Conventional, commercial hydrodesulfurization catalysts, such as cobalt-molybdenum mixtures, nickelcobalt-molybdenum mixtures, nickel-tungsten mixtures, etc. deposited upon porous carriers such as activated alumina, silica-alumina cracking catalysts which have been substantially deactivated, etc., can be used. High surface area aluminas such described in U.S. patent ap- 3 plication Serial No. 118,240, filed June 20, 1961, now Patent No. 3,188,174, Kehl and Stewart, are especially advantageous as porous carriers.

The hydrodesulfurization stage of the combination process of this invention is carried out at a temperature between about 700 and 850 F. at a hydrogen partial pressure between about 400 and 2000 p.s.i.g., preferably 750 to 1500 p.s.i.g., utilizing a space velocity (volume of charge per volume of catalyst per hour) of about 0.2 and and preferably about 0.5 and 2. The hydrogen gas which is used during the hydrodesulfuriaztion is circulated at a rate between about 2000 and 15,000 s.c.f./bbl. of feed and preferably between about 5000 and 10,000 s.c.f./bbl. The hydrogen purity may vary from about 60 to 100 percent hydrogen. If the hydrogen is recycled, which is customary, it is desirable to provide for bleeding-off a portion of the recycle gas and to add make-up hydrogen in order to maintain the hydrogen purity within the range specified. Satisfactory removal of hydrogen sulfide from the recycled gas will ordinarily be accomplished by such bleed-off procedures. However, if desired, the recycle gas may be washed or otherwise treated in known manner to reduce the hydrogen sulfide content thereof prior to recycling. The objective during hydrodesulfurization is to remove sulfur without concomitant cracking of the hydrocarbons present in the feed stock. To accomplish this, we utilize relatively mild hydrodesulfurization conditions. Thus, the temperature and space velocity are selected within the range specified which will result in a product having a pentane-insolubles content that is no greater than the pentane-insolubles content of the feed stock. The more severe the reaction conditions as regards temperature and space velocity, the higher will be the pentane-insolubles content. The pentane-insolubles content referred to is determined by the standard ASTM D-893 procedure. In general, this test measures the proportion of material that is insoluble in n-pentame and is indicative of high molecular weight, polynuclear, aromatic compounds, such as asphaltenes.

The entire hydrodesulfurized product, or alternatively, the residual portion of the hydrodesulfurized product free from furnace oil and lighter materials, is then subjected to the visbreaking stage of the combination process of this invention. We prefer first to subject the hydrodesulfurized product to distillation in order to separate or remove lower boiling products formed during this stage. Thus, some naphtha or gasoline and furnace oil will be formed during hydrodesulfurization, and these materials are advantageously removed prior to visbreaking. The visbreaking operation is carried out at a temperature between about 800 and 1000 F., and preferably between about 870 and 950 F. The pressure during visbreaking is maintained between about 50 and 1000 p.s.i.g. The coil volumes should be between about 0.012 and 0.050 cubic feet of coil volume above 750 F. per bar-rel of throughput per 24-hour day. The reaction conditions are selected within the ranges specified so as to result in less than about 20 percent formation of hydrocarbons boiling in the naphtha or gasoline range.

The product from the visbreaking operation may be subjected to distillation to remove lower boiling, more valuable materials formed during visbreaking. Thus, if the lower boiling materials formed during hydrodesulfurization were not removed before visbreaking, these materials, a well as those formed during visbreaking, may be recovered and used as naphtha, furnace oil, etc. If the low boiling materials formed during hydrodesulfurization are removed prior to the visbreaking operation, then similar materials are formed during visbreaking, and it is frequently advantageous to separate them, at least in part, from the lower value residual fraction. If these lower boiling materials are removed after visbreaking, it may be necessary to adjust the extent of removal in order to leave a residual fraction having a viscosity meeting the specifications for the residual fuel oil which is desired as a final product. While our invention may be used to produce residual fuels of any type, it is particularly advantageous for the preparation of No. 6 and/or 3500 Redwood No. 1 fuel oil. Therefore, the lower boiling components are removed to an extent sufficient to give the required viscosity for such a residual fuel or they may be removed to a greater extent and the same material or another material added as a cutter oil to reduce the viscosity.

The effectiveness of the combination process of this invention has been demonstrated by comparison thereof with conventional visbreaking of residual stocks.

Example I A Kuwait vacuum residue having the properties shown in column A of Table I was subjected to visbreaking at a temperature of 930 F., at a pressure of 200 p.s.i.g. with 0.020 cubic feet of coil volume above 750 F. per barrel throughput per day. The product from visbreaking was subjected to a distillation to remove components boiling below 400 F. and to leave a residual bottoms fraction boiling above 400 F. The properties of this product are shown in column B, Table I.

To serve as a comparison, an equivalent, substantially identical fraction of Kuwait crude, having a pentane-insolubles content of 15.1 percent, was subjected first to hydrodesulfurization alone, and thereafter the 670 F.+ fraction of this hydrodesulfurized product, having a pentane-insolubles content of 9.42 percent, was subjected to visbreaking. The products of the hydrodesulfurization run and of the run involving both hydrodesulfurization and visbreaking are shown in columns C and D, respectively. In the hydrodesulfurization operation the 1000 F.+ Kuwait vacuum residue was treated in the presence of hydrogen and in the presence of nickel-cobalt-molybdenum catalyst deposited upon activated alumina. This hydrodesulfurization was carried out at a temperature of 750 F., a hydrogen pressure of 1000 p.s.i.g., a space velocity (LHSV) of 1.0, and a hydrogen recycle rate of 10,000 s.c.f./bbl.

The product from this hydrodesulfurization was subjected to distillation as indicated above, and a bottoms fraction boiling above 670 F. was separated.

TABLE I A B C D Yields, Vol. Percent Vacuum, Tower Bottoms:

Gasoline (Cr-400 F.) 11. 5 3.8 14.6 Excess Furnace Oil (400-670 F None 6. B 6. 8 Residual Oil 90. 3 91. 6 80. 7 Inspections on Residual 0 Boiling Range, F 1,000+ 400+ 670+ 400+ Gravity, AII 6. 7 5. 9 11.4 10.3 Sulfur, Percent by Wt. 5. 2 5. 5 2. 26 2.01 Pour Point, F 100 60 80 Viscosity, SUV at 210 F. 7,080 950 1, 532 379 Fuel Oil Blends: Cutter Oil Required for 200 SFS No. 6 Fuel Oil, Percent by Vol. of Base Stoel 38. 7 22. 5 25.0 11. 0 Reduction in Cutter Oil Required for Blending to No. 6 Fuel Oil, Percent... 41.8 35.4 50.0 Reduction in Pour Point,

Comparison of the pour points and cutter oil requirements for the residual oil products in column B obtained by visbreaking, and for the residual oil obtained by hydrodesulfurization, as shown in column C, with the corresponding values in column D for the residual oil obtained by the combination process of this invention, shows that visbreaking has a markedly greater effect in these respects when carried out on a hydrodesulfurized oil. Even more remarkable is the fact that visbreaking reduced the sulfur content of the hydrodesulfurized residual oil, while visbreaking increased or concentrated the sulfur content of the straight run residual oil. Other properties are improved by the process of the present invention, thus the product of our invention has a higher gravity, lower pour point and lower viscosity. Furthermore, the amount of No. 6 fuel oil produced is considerably less when the present invention is employed, and more valuable products such as gasoline and furnace oil are considerably greater.

That the order in which the respective stages of the combination process of this invention are carried out is important for purposes of this invention has been demonstrated by comparison of the hydrogen consumed in two runs in which a straight run residual oil was first visbroken and then the product boiling above 400 F. was hydrodesulfurized, and in which the same oil was first hydrodesulfurized and the product boiling .above 670 F. was visbroken. Hydrodesulfurizing and visbreaking in each instance were carried out under the conditions described in Example I. The hydrogen consumption in the first run was 760 standard cubic feet per barrel and in the second run, which was representative of the process of this invention, the hydrogen consumption was only 600 standard cubic feet per barrel, a reduction of more than 21 percent.

That the order in which the stages of the combination process of this invention is important has also been demonstrated by comparison of the desulfurization obtained in another pair of runs involving hydrodesulfurizing a straight run residual oil with a nickelcobalt-molybdenum on alumina catalyst at 1000 p.s.i.g. hydrogen partial pressure, 0.5 liquid hourly space velocity, a temperature of 790 F., and with a hydrogen recycle rate of 10,000 standard cubic feet per barrel of oil, and hydrodesulfurizing the same oil at the same conditions, after visbreaking as described in Example I. The desulfurization obtained in the first run, which was representative of the present invention, was 84.5 percent, whereas the desulfurization obtained in the second run was 74.9 percent.

We claim:

1. A process for manufacturing heavy fuel oils of improved quality from heavy petroleum oils having objectionabl y high viscosities and sulfur contents, comprising subjecting a residual hydrocarbon charge stock that has a gravity below about 20 API and a sulfur content of at least about 2 percent, to catalytic hydrodesulfurization at a hydrogen partial pressure below about 2000 p.s.i.g., at a temperature between about 700 F. and 850 F. and at a space velocity between about 0.2 and 10, the temperature and space velocity being selected within the ranges specified to yield principally a residual product free from furnace oil and lighter materials that has a pentane-insolubles content that is not greater than the pentane-insolubles content of the charge stock, said residual product having a somewhat reduced viscosity, sulfur content and pour point and being characterized by an increased susceptibility to viscosity reduction by visbreaking, and subjecting at least the residual portion of the hydrodesulfurized product to .a visbreaking treatment at a temperature between about 800 F. and 1000 F., a pressure between about 50 and 1000 p.s.i.g., and with a coil volume of between about 0.012 and 0.050 cubic feet of coil volume above 750 F. per barrel of throughput per 24-hour day, the combination of conditions being selected to form below about 20 percent 400 F. end point gasoline and, so as to yield principally a heavy fuel oil product having still further reduced viscosity, pour point and sulfur content, and that requires reduced quantities of cutter oil to obtain an acceptable viscosity for use as a heavy fuel oil.

2. The process for manufacturing heavy fuel oils of improved quality from heavy petroleum oils having objectionably high viscosities and sulfur contents that comprises subjecting a straight run residual fraction of petroleum that has a gravity below about 20 API and a sulfur content of at least about 2 percent, to catalytic hydrodesulfurization at a hydrogen partial pressure below about 2000 p.s.i.g., at a temperature between about 700 F. and 850 F. and at a space velocity between about 0.2 and 10, the temperature and space velocity being selected within the ranges specified to yield principally a product free from furnace oil and lighter materials that has .a pentane-insolubles content which is below about the pentane-insolubles content of the charge stock, said residual product having a somewhat reduced viscosity, sulfur content and pour point and being characterized by an increased susceptibility to viscosity reduction by visbreaking, and subjecting at least the residual portion of the hydrodesulfurized product to a visbreaking treatment at a temperature between about 800 F. and 1000 F., a pressure between about 50 and 1000 p.s.i.g., and with a coil volume of between about 0.012 and 0.050 cubic feet of coil volume above 750 F. per barrel of throughput per 24-hour day, the combination of conditions being selected to form below about 20 percent 400 F. end point gasoline and, so as to yield principally a heavy fuel oil product having still further reduced viscosity, pour point and sulfur content, and that requires reduced quantities of cutter oil to obtain an acceptable viscosity for use as a heavy fuel oil.

3. The process for manufacturing heavy fuel oils of improved quality from heavy petroleum oils having objectionably high viscosities and sulfur contents, which comprises subjecting a straight run residual petroleum fraction which has a gravity below about 20 API and a sulfur content of at least about 2 percent, a catalytic hydrodesulfurization at a hydrogen partial pressure between about 750 and 1500 p.s.i.g., at a temperature between about 750 F. and 850 F. and at a space velocity between about 0.5 and 2, the temperature and space velocity being selected within the ranges specified to yield principally :a product free from furnace oil and lighter materials that has a pentane-insolubles content which is below about the pentane-insolubles content of the charge stock, said residual product having a somewhat reduced viscosity, sulfur content and pour point and being characterized by an increased susceptibility to viscosity reduction by visbreaking, and subjecting at least the residual portion of the hydrodesulfurized product to a visbreaking treatment at a temperature between about 800 F. and 1000 F.. a pressure between about 50 and 1000 p.s.i.g., and with a coil volume of between about 0.012 and 0.050 cubic feet of coil volume above 750 F. per barrel of throughput per 24-hour day, the combination of conditions being selected to form below about 20 percent 400 F. end point gasoline and, so as to yield principally a heavy fuel oil product having still further reduced viscosity, pour point and sulfur content, and that requires reduced quantities of cutter oil to obtain an acceptable viscosity for use as a heavy fuel oil.

References Cited UNITED STATES PATENTS 1,932,174 10/1933 Gaus et a1 20889 2,235,366 8/1944 Conn 208-89 2,871,182 1/1959 Weekman 208-57 DELBERT F. GANTZ, Primary Examiner. S. P. JONES, Assistant Examiner. 

1. A PROCESS FOR MANUFACTURING HEAVY FUEL OILS OF IMPROVED QUALITY FROM HEAVY PETROLEUM OIL S HAVING OBJECTIONABLY HIGH VISCOSITIES AND SULFUR CONTENTS, COMPRISING SUBJECTING A RESIDUAL HYDROCARBON CHARGE STOCK THAT HAS A GRAVITY BELOW ABOUT 20* API AND A SULFUR CONTENT OF AT LEAST ABOUT 2 PERCENT, TO CATALYTIC HYDRODESULFURIZATION AT A HYDROGEN PARTIAL PRESSURE BELOW ABOUT 2000 P.S.I.G., AT A TEMPERATURE BETWEEN ABOUT 700*F. AND 850* F. AND AT A SPACE VELOCITY BETWEEN ABOUT 0.2 AND 10, THE TEMPERATURE AND SPACE VELOCITY BEING SELECTED WITHIN THE RANGES SPECIFIED TO YIELD PRINCIPALLY A RESIDUAL PRODUCT FREE FROM FURNACE IL AND LIGHTER MATERIALS THAT HAS A PENTANE-INSOLUBLES CONTENT THAT IS NOT GREATER THAN THE PENTANE-INSOLUBLES CONTENT OF THE CHARGE STOCK, SAID RESIDUAL PRODUCT HAVING A SOMEWHAT REDUCED VISCOSITY, SULFUR CONTENT AND POUR POINT AND BEING CHARACTERIZED BY AN INCREASED SUSCEPTIBILITY TO VISCOSITY REDUCTION BY VISBREAKING, AND SUBJECTING AT LEAST THE RESIDUAL PORTION OF THE HYDRODESULFURIZED PRODUCT TO A VISBREAKING TREATMENT AT A TEMPERATURE BETWEEN ABOUT 800*F. AND 1000*F., A PRESSURE BETWEEN ABOUT 50 AND 100 P.S.I.G., AND WITH A COIL VOLUME OF BETWEEN ABOUT 0.012 AND 0.050 CUBIC FEET OF COIL VOLUME ABOVE 750*F. PER BARREL OF THROUGHPUT PER 24-HOUR DAY, THE COMBINATION OF CONDITIONS BEING SELECTED TO FORM BELOW ABOUT 20 PERCENT 400*F. END POINT GASOLINE AND, SO AS TO YIELD PRINCIPALLY A HEAVY FUEL OIL PRODUCT HAVING STILL FURTHER REDUCED VISCOSITY, POUR POINT AND SULFUR CONTENT, AND THAT REQUIRES REDUCE QUANTITIES OF CUTTER OIL TO OBTAIN AN ACCEPTABLE VISCOSITY FOR USE AS A HEAVY FUEL OIL. 